In conventional tape cartridges for printers and typewriters and the like, the end of tape feed can be sensed by detecting the passage of a reflective element at the end of the tape past a sensor. At such time a reflective pulse is produced from a light source and the sensor acts to produce an electrical signal which halts the drive of the tape and carrier.
As disclosed in U.S. Pat. No. 4,115,013, a sensor can be fixed on the body of a carrier and the cartridge can be provided with an opening into which the sensor fits when the cartridge is mounted on the carrier. The position of the sensor is such that the light source projects a beam on the tape and at the end of tape feed, and a reflective element comes into a position where it reflects the light back to the sensor to generate a signal which halts the tape feed and printer.
This conventional system is effective and reliable and has found widespread usage. However, it has the deficiency that if the tape breaks, the tape drive continues and the printer operates without producing any printing due to the absence of a tape supply.
According to another known system, a mechanical flag is oscillated as the tape supply reel rotates and the flag is positioned between a light source and a sensor. Hence, as the supply reel continues to rotate, an intermittent pulse is produced by the sensor to indicate such continuous rotation. In the absence of a continuous train of pulses, the drive is halted.
This system has the advantage that if there is a break in the tape, the drives of the carrier and tape will be halted. However, the system has the disadvantage of using mechanical connections which are detrimental to the smooth rotation of the supply reel and can lead to breakdowns.
In another system, the opaque tape passes between a light source and a receiver to block passage of light therebetween and, at the end of the tape, a transparent portion is provided so that light can pass from the source to the receiver. Hence, at the end of the tape, a signal is produced to halt the tape feed and carrier drive. Should the tape break downstream of the light source and receiver, the carrier drive will continue to operate as will the tape feed despite the fact that no tape is being fed. If the tape breaks upstream of the light source and receiver, the loose length of tape will continue to be pulled by the tape feed until it passes through the light source and receiver whereupon the tape feed and carrier drive will be halted. This system lacks certainty in detecting tape breakage and unless breakage takes place precisely at the light source and receiver (a highly unlikely occurrence), either there will be no detection (downstream breakage) or there will be a lengthy delay in the detection (upstream breakage).